1. Field of the Invention
The present invention relates in general to a geomagnetic sensor for detecting a dip angle and a method thereof, more particularly, to a geomagnetic sensor for detecting a present dip angle through a simple operation using a 2-axis fluxgate and a method thereof.
2. Description of the Related Art
A geomagnetic sensor is an instrument for measuring the intensity and direction of a geomagnetic field which human beings are not able to sense. A geomagnetic sensor using a fluxgate is called a geomagnetic fluxgate sensor.
In particular, the geomagnetic fluxgate sensor has a magnetic core made from high permeability materials, such as permalloy. To measure the intensity and direction of an external magnetic field, an induced magnetic field is applied through winding the magnetic core with coils, and second harmonic components proportional to the external magnetic field that varies depending on magnetic saturation of the magnetic core and non-linear magnetic
The magnetic fluxgate sensor was first developed at the end of 1930's. Compared with other geomagnetic sensors, it features high sensitivity, low manufacture cost, small size, low power consumption, and long-term stability of output signals. Because of these merits, the magnetic fluxgate sensor has been broadly used not only for military use, but also for civilian demands, such as, detection of a weak magnetic field, measurement of the absolute direction of the Earth's magnetic field, exploration of a vein of ore, target detection, control of a satellite's position, and space exploration. Studies on the improvement of the magnetic fluxgate sensor's performance are still in progress.
The recent development of Micro-Electro-Mechanical Systems (MEMS) technology has served to motivate the development of a low-power consumption, microminiature fluxgate sensor.
FIG. 1 is a schematic block diagram of a related art geomagnetic sensor. As shown in FIG. 1, the geomagnetic sensor 10 includes a drive signal generator 11, a geomagnetic detection module 12, a signal processor 13, a controller 14, a memory 15, and an accelerometer module 16.
The drive signal generator 11 generates a signal for driving the geomagnetic detection module 12. Usually, pulse-type and inverted pulse-type drive signals are used.
The geomagnetic detection module 12 includes two fluxgates that are orthogonal to each other. Each fluxgate includes a rectangular ring-shaped or bar-shaped magnetic core, a drive coil wound around the core, and a detection coil. The drive coil receives an electrical drive signal outputted from the drive signal generator 11, and magnetizes the core. The detection coil detects an electromotive force induced from the magnetic field generated by the drive coil.
In the meantime, if a voltage component proportional to the intensity of an external magnetic field is induced in the detection coil, the signal processor 13 amplifies and chops the voltage component, and then outputs a voltage value corresponding to each axis.
The controller 14 measures the voltage value outputted from the signal processor 13, and the geomagnetic sensor's azimuth at present. In general, an electrical signal outputted from each fluxgate of the geomagnetic detection module 12 changes according to the change in tilt angle of the geomagnetic sensor. If the change in the tilt angle is great, the azimuth measurement can be erroneous. In such case, the effect of the tilt needs to be calibrated on the basis of the tilt calibration algorithm. To calibrate the effect of the tilt according to the tilt calibration algorithm, the tilt of the present geomagnetic sensor 10, i.e., pitch angle and roll angle, should be measured first, and a dip angle at the present position of the geomagnetic sensor 10 is measured next.
The accelerometer module 16 measures the acceleration of gravity of the geomagnetic sensor 10, and computes the pitch angle and the roll angle. Similar to the geomagnetic detection module 12, the accelerometer module 16 includes two orthogonal accelerometers. The pitch angle and the roll angle indicate how much the geomagnetic sensor 10 is tilted, that is, the tilt angle of the geomagnetic sensor 10. More specifically, the pitch angle is the angle of rotation between the plane containing the geomagnetic sensor 10 and one of two orthogonal axes (X-axis and Y-axis). The roll angle is the angle of rotation between the plane containing the geomagnetic sensor 10 and the other axis.
To measure the pitch angle and the roll angle, the accelerometer module 16 usually employs a weight having a constant mass in order to check the motion of the weight against the force of gravity visually or through an angle meter, a rulers or an indication needle. The pitch angle and the roll angle measurements are stored in the memory 15.
In case of compensating the tilt, the controller 14 normalizes voltage values of the X-axis and the Y-axis outputted from the signal processor 13, and computes the azimuth by using the pitch angle and the roll angle measured by the accelerometer module 16, and the dip angle at the present position. Here, dip angle is one of three elements of the terrestrial magnetism (declination, dip angle, and horizontal magnetic force), and is the angle between a magnetic needle and a horizontal plane as the magnetic needle freely rotates on a vertical plane. For instance, the dip angle of Korea ranges approximately from 50° to 60°.
Traditionally, a specific dip angle was given to each geographic area, or a dip angle was inputted from an external device, e.g., a Global Positioning System (“GPS”), for the calculation of azimuth. Using the dip angle designated per area is effective if the area is not changed. Otherwise, a new dip angle should be inputted every time the area changes. Also, even within the same area, the intensity of the Earth's magnetic field is distorted and thus, the designated dip angle can be in error. On the other hand, to receive the dip value from the external device, e.g., the GPS, an additional equipment for communicating with the external device is required. Therefore, the size and manufacturing cost of the geomagnetic sensor are increased.